BST_PROFILE should complain when the required profile doesn't exist
Summary
Currently, if you run buildstream with BST_PROFILE=nonexistent
, everything will be fine, and you'll end up with no profiles at the end, which is surprising and unintuitive.
Steps to reproduce
Run BST_PROFILE=blah bst build
What is the current bug behavior?
No profile is created, nothing complains
What is the expected correct behavior?
We should at least get an warning, preferrably an error